1. Field of the Invention
The invention relates generally to the field of materials science and engineering. More particularly, the invention relates to the condensed phase conversion and growth of nanorods and other materials.
2. Discussion of the Related Art
Carbon nanotubes are stretched versions of hollow fullerenes and can be thought of as fibers formed of perfectly graphitized closed seamless shells, with unique mechanical and electronic features sensitive to their geometry and dimensions. Carbon nanotubes were discovered by Dr. Sumio lijima, a researcher specializing in electron microscopy at NEC's R&D group in Ibaraki, Japan, in 1991, while working on “buckyballs” or buckminsterfullerenes. Since this discovery, several groups have demonstrated the synthesis of various carbon nanotubes and have shown methods for collection, purification and incorporation of these carbon nanotubes in small components and composite structures.
Carbon nanotubes are a relatively new, elegantly geometrical material which can ignite a revolution in electronics, computers, chemistry, automotive, aerospace, defense programs and a myriad of structural systems. However, these improvements can only be realized if the manufacturing development aspects are addressed to “leap-frog” the present state of the art. All previous methods for forming nanotube materials are by vapor phase processes.
Heretofore, growth mechanisms for the formation of carbon nanotubes, both single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs), have not been understood. This has prevented the required “leap-frog” scaling to manufacturing technology for large structures.
Carbon nanotubes include both multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). MWCNTs have exhibited ballistic conductance at room temperature. Ballistic conductance is a phenomenon in which electrons pass through a conductor without heating. A commercial approach to fabricating MWCNT would enable atomic and molecular-sized electronic devices that offer unique applications.
Single-walled carbon nanotubes (SWCNT) were discovered in 1993. Although SWCNT are currently only produced in small quantities, their remarkable electrical properties are rapidly being developed for commercial applications nearly as quickly as they are discovered. SWCNT are as small as 1 nm in diameter and can be up to 3 cm long. SWCNT have also exhibited ballistic transport (lossless current propagation) when used as metallic molecular wires.
SWCNT have the potential to add tremendous capability and functionality to future systems. Components fabricated from SWCNT would represent the strongest possible structural material with other unique physical properties. SWCNT structural materials would posses a strength to weight ratio of 812 to 1 over aluminum and 731 to 1 over titanium. SWCNT have already shown conductivities greater than copper at room temperature. SWCNT have exhibited both high thermal and electrical conductivities and could provide unique low observable components. Incredible potential for increased ballistics survivability if not invulnerability appears quite possible.
In addition, SWCNTs also possess other unique properties which hold even greater, almost unimaginable, economic opportunities and importance for our national energy and defense goals. SWCNT are the strongest material known to man, with over 1 TPa Young's modulus in the axial direction. Bundled SWCNT are predicted to have the largest strength-to-weight ratio of any known material, and promise new generations of lightweight, supertough structural materials which could replace metals in the bodies and engines of automobiles, aircraft, and ships, as well as form a new class of energy-efficient building materials. Single-walled carbon nanotubes are also highly thermally conductive, can withstand high temperatures, and are resistant to even strong acids. These features make them extremely desirable for aerospace applications. The weight savings benefits from SWCNT-construction would enable higher payloads for the next generation of space shuttles and airplanes. Alternatively, cables from SWCNT are predicted to be strong enough to hoist payloads from the earth's surface to orbiting space stations. Finally, SWCNT recently exhibited 8 wt. % hydrogen sorption (the highest for any carbon material) which make them desirable for hydrogen storage fuel cells for clean cars of the future.
Currently, structural applications of SWCNTs incorporate them in conventional fiber/epoxy systems. The SWCNT must be created, collected, purified, and then mixed with a matrix material for the production of composite structures (i.e., polymeric or resin composite structures).
By doing this, SWCNTs have not reached their full structural application potential. Limitations in the mechanical properties of the epoxy resins and costs associated with manufacturing using the fabrication processes associated with fiber/epoxy systems severely limit the capability and application of SWCNTs.
Currently SWCNTs are produced in laboratory-scale environments by 3 techniques at maximum rates of 16 grams/day. SWCNTs are produced by laser vaporization (LV) (approximately 1 g/day), electric-arc vaporization (AV) (less than 100 g/day) and most recently by chemical vapor deposition (CVD) (growth rates of 100 μm/hour, albeit over large areas). The LV and AV methods produce loose nanotubes which are grown in the gas-phase from co-vaporized carbon and approximately 1% catalyst metal. CVD utilizes thermal decomposition of a mixture of carbon-containing and metal-catalyst-containing precursor gases (e.g., methane and ferrocene) above a hot substrate. These methods are not suitable for direct fabrication of structural components. Minimum production rates of several kg/hour (roughly a factor of 1000 improvement) must be achieved for cost-effective replacement of current structural materials in high value-added products.
While recipes have been phenomenologically developed for synthesis of SWCNTs by each of the three methods described above, it is noteworthy that virtually no in situ diagnostics have been developed to characterize the growth process. As a result, the growth process is not understood and, consequently, these processes are not optimized.
In April 1999, a symposium was held in Washington, D.C. to address the problem of large-scale production of carbon nanotubes. Two conclusions were emphasized: First, the synthesis process must be understood and in situ diagnostics must be developed to help optimize it. Second, a high-volume industrial process must be developed at low cost.